1. Field of the Invention
The present invention is directed to a process of drying semiconductor wafers employing liquid or supercritical carbon dioxide. More specifically, the present invention is directed to a process of drying semiconductor wafers characterized by the presence of surfaces having high aspect ratios utilizing a drying composition which includes liquid or supercritical carbon dioxide.
2. Background of the Prior Art
The processing of semiconductor wafers, which are subsequently cut into semiconductor devices, involves several processing procedures such as photoresist development and wet etching. These processing procedures all include the step of removing by-product chemical species by water rinsing. With the advent of smaller and smaller sized semiconductor chips the subsequent step of drying the semiconductor wafer to remove all trace of rinse water and any dissolved species dispersed, dissolved or emulsified therewith has become an increasingly difficult operation.
Older methods of water removal from semiconductor wafers, which were successful in the past, because of the relatively large size of the structures formed on semiconductor wafers, are no longer effective in removing water remaining after rinsing. This is because of the inclusion, on the semiconductor wafer, of high aspect ratio holes, indentations, vias and the like, e.g. height to width ratios of 2 or more. In view of the relatively high surface tension of water, water removal from such structures poses a difficult technological problem. Indeed, water removal can damage the electrical assembly structure.
The aforementioned problem is appreciated in the recent prior art. Thus, new methods of drying semiconductor wafers, without the utilization of heat or turbulent gas flow, which overcomes the surface tension problems discussed above, but which create new problems involving residues, have been suggested. For example, the use of ultrasonic or megasonic energy in the removal of liquid medium is disclosed in U.S. Pat. Nos. 4,788,992; 5,333,628; and 5,339,842.
Another method of drying semiconductor substrates advanced in the prior art involves the use of so-called xe2x80x9cvapor dryersxe2x80x9d. In this method a vapor, of which isopropanol and fluorinated hydrocarbons are preferred, is condensed on the surface of a semiconductor wafer to flush aqueous liquid therefrom. Examples of this method are provided in U.S. Pat. Nos. 5,055,138; 5,183,067; 5,243,768; 5,351,419; and 5,371,950.
More recently, as semiconductors have become more and more miniaturized, even reaching the nanometer size range, xe2x80x9cMarangonixe2x80x9d type drying has been advanced. In this procedure a solvent vapor is applied to an aqueous film, which lowers the surface tension of the liquid film, inducing the aqueous liquid film to leave the surface of the wafer. Such a process is described in U.S. Pat. No. 5,660,642.
A still further improvement in drying of semiconductor wafers is provided by U.S. Pat. No. 6,067,728. The ""728 patent advances a method in which a semiconductor wafer is submerged in methanol and thereupon contacted with supercritical carbon dioxide. Although this method represents an improvement over earlier processes of drying semiconductor wafers, it is costly and environmentally unfriendly insofar as methanol, which is highly flammable, is released to the atmosphere.
The above description of prior art methods of drying semiconductor wafers suggests the need in the art for a further improved process for removing residual water especially in those wafers provided with topographical structures having high aspect ratios.
A new process has now been discovered which provides significant improvement in the removal of water from semiconductor wafers. Although the present invention is independent of any theory explaining its operation, it is postulated that the process of the present invention represents a significant improvement over prior art methods insofar as the present process results in significant reduction of interfacial tension between the extracting fluid and the film of water which is sought to be removed. In addition, the process of the present invention increases coalescence or dissolution of water in the extracting fluid.
In accordance with the present invention a process of drying a semiconductor wafer having a microelectronic structure thereon with a composition which comprises liquid or supercritical carbon dioxide and a surfactant wherein the surfactant is present in a concentration of between about 0.1% and about 50% by weight, based on the total weight of the composition, is provided.